Flowing water pushes on the paddles, turning the wheel. The paddles have a rubber strip on their outside edges. Somehow debris gets depositied onto the paddle, perhaps by being scraped off the bottom of the box or off the surface of a hidden "vertical" screen. As the paddle rises, the debris falls off onto the shelf, where it collects. I think the shelf had a screen to allow water to flow through. I’m writing this eight years after taking the pictures and I cannot remember how the shelf gets cleaned. Maybe it periodically tilts to dischagrge its load.